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  <title>update (Alien)</title>
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  <pre><span class="ruby-comment cmt"># File alien.rb, line 25</span>
  <span class="ruby-keyword kw">def</span> <span class="ruby-identifier">update</span>
    <span class="ruby-comment cmt"># move</span>
    <span class="ruby-ivar">@x</span> <span class="ruby-operator">+=</span> <span class="ruby-ivar">@speedx</span>
    <span class="ruby-ivar">@y</span> <span class="ruby-operator">+=</span> <span class="ruby-ivar">@speedy</span>
    
    <span class="ruby-comment cmt"># destroy when the alien reaches the bottom of the screen</span>
    <span class="ruby-identifier">kill!</span> <span class="ruby-keyword kw">if</span> <span class="ruby-ivar">@y</span> <span class="ruby-operator">&gt;</span> <span class="ruby-constant">Engine</span><span class="ruby-operator">::</span><span class="ruby-constant">Game</span><span class="ruby-operator">::</span><span class="ruby-constant">ScreenHeight</span> <span class="ruby-operator">+</span> <span class="ruby-ivar">@image</span>[<span class="ruby-value">0</span>].<span class="ruby-identifier">height</span> <span class="ruby-operator">/</span> <span class="ruby-value">2</span>
    
    <span class="ruby-comment cmt"># animation</span>
    <span class="ruby-ivar">@index</span> = <span class="ruby-constant">Gosu</span><span class="ruby-operator">::</span><span class="ruby-identifier">milliseconds</span> <span class="ruby-operator">/</span> <span class="ruby-constant">AnimTime</span> <span class="ruby-operator">%</span> <span class="ruby-ivar">@image</span>.<span class="ruby-identifier">size</span>
  <span class="ruby-keyword kw">end</span></pre>
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